Non-contact position sensor

ABSTRACT

A device for measuring the displacement of a movable object includes a stationary light source ( 10 ) that produces an incident light beam. A target feature ( 17 ), attached to, or integral with, the object, reflects the incident light beam and forms a first image of the light source in close proximity of the target feature. An imaging lens ( 26 ) receives the reflected light beam and reforms the first image of the light source as a second image on the photodetector ( 20 ). The photodetector, spaced from the object, receives the reflected light beam and produces an electric signal having a characteristic which is proportional to a received location on the photodetector of the second image and which represents a position of the object. The target feature includes a curved surface that reflects the light beam such that a small, point-like or line-like first image of the light source is formed and reformed as a second image on the photodetector.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus for accurately measuring minute displacements of a movable object and, more particularly, to a simple, yet precise, optical method and apparatus for providing such displacement measurements.

BACKGROUND OF THE INVENTION

There is a great need in many applications to monitor the positions of a movable object relative to a desired position or positions. For example, it is desirable in disk drive servo systems to accurately detect the positions of a rotatable read/write head arm, relative to desired, known positions, so that the head arm can be moved into alignment with desired radial track locations on a disk to enable the read/write head to read from and write to such locations. Position sensing devices are devices aimed at providing such position detection. Displacement sensors are position sensing devices that monitor the positions of a movable object by repeatedly measuring the displacements of the object from a desired location.

Examples of conventional displacement sensors include capacitance gage devices, fiber-optic proximity sensors and optical sensors, such as interferometric sensors used commonly in disk drive servo systems. Many of the prior art displacement sensors either are incapable of achieving the degree of resolution (the smallest measureable displacement) required by certain applications (such as disk drive servo systems) or require expensive and/or complex circuitry and hardware to achieve such resolution. To achieve high resolution, certain displacement sensors require, for example, a precise and powerful laser light source or extremely close proximity between the sensing element and movable object, rendering such devices expensive and difficult to implement. Thus, a balance must be struck between device performance and simplicity.

U.S. Pat. No. 5,315,372 to Tsai describes a prior art disk drive servo system that employs an optical displacement sensor. The Tsai device includes a light source and a photodetector array attached, at spaced-apart locations, to a rotatable master arm, located externally of the disk drive. A reflector is attached to a rotatable read/write head arm at a location between the read/write head and the axis of rotation of the arm. During operation, the master arm first is aligned accurately with a desired radial track location on the disk using an interferometric device. Then, the displacement sensor operates to determine the position of the head arm relative to the master arm so that the head arm can be moved into alignment with the master arm.

The light source of the displacement sensor produces an incident light beam that is reflected by the reflector onto the photodetector array. The position on the photodetector array to which the light beam is reflected depends on the relative radial positions of the head and master arms. Each photodetector element of the array produces an electric signal having an amplitude proportional to the intensity of the received light. The signals produced by the array, thus, represent the relative radial positions of the head and master arms. Processing circuitry receives and decodes the signals output by the array to determine the relative head arm position and accordingly controls a motor to rotate the head arm until it is properly aligned.

The device disclosed in the Tsai patent suffers from a number of drawbacks. While the Tsai device is relatively structurally simple and fairly inexpensive to implement, it is burdensome to operate. The Tsai device requires a pre-knowledge of each master arm and relative head arm position in order to accurately interpret the electric signals produced by the photodetector array. Also, due to the significant spacing between the reflector and the read/write head on the head arm and, due also to the dual axes of rotation of the head arm and master arm, performance accuracy is sacrificed severely. Further, the displacement sensor of Tsai aims to monitor radial movements (those caused by rotations about the axis) of the head arm relative to the master arm. Because the reflector reflects the incident beam directly onto the photodetector, spaced significantly from the reflector, the sensor is sensitive not only to radial movements of the head arm but also to angular movements of the reflector. Thus, the precise angular orientation of the reflector on the head arm is crucial to precise operation. Any angular movements of the reflector with respect to the head arm may cause false measurements to occur.

It accordingly is a general object of the present invention to provide a simple, yet accurate, displacement sensor.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a device for measuring the displacement of a movable object. The device includes a stationary light source that produces an incident light beam. A target feature, attached to, or integral with, the object, reflects the incident light beam and forms a first image of the light source in close proximity to the target feature. The first image moves with the target feature. An imaging lens receives the reflected light beam and forms a second image of the light source on a stationary photodetector. The photodetector, spaced from the object, receives the second image and, in response thereto, produces an electric signal having a characteristic (such as amplitude) which is proportional to an instantaneous position on the photodetector of the second image and which represents the position of the object. The target feature includes a curved surface that reflects the light beam such that a small, preferably point-like or line-like first image of the light source is formed near the target feature and is reformed as a second image on the photodetector. As a result, the device of the invention measures only lateral movements of the object and is insensitive to other movements such as the angular orientation of the object.

In one embodiment of the invention, the photodetector includes a spatially arranged photodetector, such as a bi-cell photodetector. In another embodiment of the invention, the photodetector includes a position sensor.

In an embodiment of the invention, the target feature preferably has a radius of curvature within the range of 0.2-0.5 mm.

In an embodiment of the invention, the incident light beam and the reflected light beam form a plane substantially perpendicular to the direction of the motion to be sensed. In that embodiment, an axis formed through a center point of each cell of the bi-cell detector is substantially orthogonal to the plane and substantially parallel to a direction of motion of the object.

Another embodiment of the invention is directed to an optical displacement sensor for use in a disk drive servo system. The sensor monitors the relative radial positions of a read/write head arm and a master arm. The device includes a light source, attached to the master arm, that produces an incident light beam. A target feature, attached to, or integral with, an end of the head arm, reflects the incident light beam and forms a first image of the light source in close proximity to the target feature. The first image moves with the target feature. An imaging lens receives the reflected light beam and forms a second image of the light source on a photodetector. The photodetector, attached to the master arm, receives the second image and, in response thereto, produces an electric signal having an amplitude which is proportional to an instantaneous position on the photodetector of the second image and which represents a relative position of the head arm. The target feature includes a curved surface that reflects the light beam such that a small, preferably point-like or line-like first image of the light source is formed near the target feature and is reformed as a second image on the photodetector. As such, the displacement sensor is sensitive only to lateral movements of the target feature and is insensitive to other movements. Thus, the angular orientation of the target feature on the head arm will not affect measurements and need not be precise.

A further embodiment of the invention is directed to a method of measuring the displacement of a movable object. The method includes the steps of: producing an incident light beam with a stationary light source; reflecting the incident light beam from a curved surface of a target feature to form a first image of the light source in close proximity of the target feature, attached to, or integral with, the object; small, reforming the first image of the light source as a second image thereof onto a stationary photodetector; and producing, with the photodetector, an electric signal having an amplitude which is proportional to each instantaneous position on the photodetector of the second image and which represents a position of the object.

The features and advantages of the present invention will be more readily understood and apparent from the following detailed description of the invention, which should be read in conjunction with the accompanying drawings, and from the claims which are appended to the end of the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a general embodiment of the displacement sensor of the present invention;

FIG. 2 is a more detailed diagram of a specific embodiment of the displacement sensor of the present invention;

FIG. 3 is a graph illustrating an electric signal produced from the outputs of a bi-cell photodetector according to one embodiment of the present invention;

FIG. 4 is a detailed block diagram of a disk drive servo system according to another embodiment of the present invention;

FIG. 5 is a part structural, part functional block diagram illustrating operation of the disk drive servo system of the present invention;

FIGS. 6a and 6 b are electrical block diagrams of the processing circuitry of the disk drive servo system of the present invention;

FIG. 7 is a detailed diagram of one embodiment of the target feature employed in the disk drive servo system of the present invention;

FIG. 8 is a diagram of the side view of the target feature and head arm tip shown in FIG. 7; and

FIG. 9 is a more detailed diagram of the portion of an embodiment of the displacement sensor of the present application.

DETAILED DESCRIPTION

FIG. 1 is a general block diagram of one embodiment of the displacement sensor of the present invention. As shown, the sensor includes a stationary light source 10, a movable object 14, a stationary imaging lens 26 and a stationary photodetector 20. A target feature 16 is attached to, or integral with, movable object 14. Object 14 is movable in the direction shown by arrow X and the displacement sensor monitors the position of object 14 by measuring displacements of object 14 from a reference position.

Light source 10 produces an incident light beam 12 which is reflected by target feature 16 as a reflected light beam 18. A first point-like or line-line image (not shown) of the source 10 is formed in close proximity of target feature 18 due to a small curved reflective surface thereon. The first image moves with movement of the target feature. Light beam 18 is focussed by lens 26 such that a second image 19 (like the first image) of the light source is formed on a surface of photodetector 20. Photodetector 20, in response to receipt of the reflected light beam 18 (and second image), produces an electric signal having a characteristic (such as amplitude or frequency) that is proportional to the position on the surface of photodetector 20 at which the reflected light beam is received (i.e., where the second image is formed) and, therefore, also corresponds to the present position of the object 14. Processing circuitry (a particular embodiment of which is described below with reference to FIG. 6B) can be connected to the photodetector to receive the electric signals produced by the photodetector and process such signals to determine the position of the object and/or displacement of the object from a known reference position. The circuitry can include both analog and digital signal processing circuitry.

As described in greater detail with reference to FIG. 9, target feature 16 includes a curved surface 17 which reflects incident light beam 12 such that a first small, preferably point-like or line-like, image of source 10 is formed in close proximity of the target feature and is reformed by lens 26 as a second image onto the surface of photodetector 20. Photodetector 20 is increasingly sensitive to smaller displacements as the size of the image is decreased. It, therefore, is desirable to refocus a small, point-like or line-like, image of the source onto the photodetector surface.

As will be described in greater detail below, curved surface 17 of target feature 16 can be concave or convex. Target feature 16 can be formed integral with object 14 using a suitable stamping tool. For example, target feature 16 can include a curved surface that is stamped into a portion of object 14. Alternatively, target feature 16 can be a separate element that is affixed to object 14. For example, the target feature 16 can be a cylindrical element such as a pin that is welded, brazed, soldered, screwed or otherwise affixed to object 14. Curved surface 17 of target feature 16 should be sufficiently smooth and preferably polished so that the incident light beam is efficiently reflected.

Referring to FIG. 1, as object 14 is moved in the direction of the arrow X, the position on the surface of photodetector 20 where reflected beam 18 is received moves in the opposite direction, shown by arrow X′. Photodetector 20, in response, produces electric signals wherein each signal has a characteristic (i.e., amplitude or frequency) that is proportional to the position on the photodetector surface of reflected beam 18 and, thus, also to the relative position of object 14.

The displacement resolution of the sensor increases as the size of the image of the light source reflected onto the photodetector surface decreases. Additionally, the displacement resolution of the sensor increases as the intensity of the incident light beam increases. Because the use of a target feature having a curved surface greatly reduces the size of the light source image (i.e., to a point-like or line-like image) reflected onto the photodetector surface, the requirement for a highly efficient and expensive laser light source is greatly relaxed in order to achieve a high degree of displacement resolution.

FIG. 2 is a more detailed diagram showing a particular embodiment of the displacement sensor of the present invention. Like elements in FIG. 2 are referred to by identical reference characters to those in FIG. 1. As shown, the sensor includes laser diode light source 22, object 14, to which target feature 16 is attached, and bi-cell photodetector 28. Also included are a gradient index (GRIN) incident laser diode beam collimating lens 24 and reflected beam focussing lens 26.

Laser diode 22 conventionally produces incident laser beam 12 (shown in FIG. 2 as two separate laser beams) which is provided through GRIN collimating lens 24 to target feature 16. Each of laser diode 22, GRIN collimating lens 24 and bi-cell photodetector 28 can be conventional elements. Incident laser beams 12 reflect from curved surface 17 of target feature 16 to form a first image (not shown) of the source in close proximity to the target feature and reflects incident beams 12 as reflected beams 18. The small, point-like or line-like first image is reformed by lens 26 as a second image 30 on upper surface 31 of bi-cell photodetector 28. Lens 26 receives beams 18 and focusses the second image 30 onto photodetector 28. Curved surface 17 of target feature 16 enables a small, point-like or line-like image 30 of the light source 22 to be refocussed on surface 31 of photodetector 28.

Because a first point-like image is formed in close proximity of the target feature, the system of the invention is significantly insensitive to movements of the object and target feature (such as radial movements) not sought to be measured. For example, if object 14 rotates slightly about an axis formed in the Y direction (parallel to the longitudinal axis of the target feature), the measurement will not be affected. This is so because the portion of reflected beams 18 (and first image) on the surface of lens 16 will change but the position of second image 30 on photodetector 31 will not change. Thus, the system of the invention is insensitive to small radial movements of the object.

Bi-cell detector 28 includes two photodetector cells A and B separated by narrow strip 32. As object 14 moves in direction X, point image 30 moves across surface 31 of bi-cell detector 28 in the opposite direction X′ from cell A to cell B. The center of point image 30 is shown in FIG. 2 as being located on strip 32 between cells A and B. The sensor could be calibrated to have a null or reference position (a position in which object 14 is at a desired location) when the center of point image 30 is focussed on strip 32 between cells A and B.

Each cell A or B of bi-cell photodetector 28 produces an analog electric signal having an amplitude that depends upon the location of the point image 30 on the cell surface. Each cell produces the strongest electric signal when the point image 30 is located wholly upon the surface of that cell. By monitoring the ratio of (A−B)/(A+B), where A and B respectively denote the amplitudes of the electric signals produced by cells A and B of detector 28, the position of point image 30 on the surface of photodetector 28 can be monitored and, thereby, the position of object 14 (relative to the stationary light source and photodetector) also can be monitored. This is so because the position of the point image 30 on the surface of photodetector 28 is linearly related to the relative position of object 14. The ratio of (A−B)/(A+B) can be monitored with the use of processing circuitry (see FIG. 6B) that would be electrically connected to the outputs of bi-cell detector 28. Such circuitry could include digital signal processing circuitry that could be connected to a computer having a display such that a user could easily visually monitor the position of object 14.

The sensor is insensitive to movements in the Y, Y′, Z or angular (about the Y axis) directions. In this embodiment, in order to reduce the sensitivity of the sensor to vertical movements (away from or toward the laser diode and photodetector) of the object, the plane formed by the incident laser beam 12 and reflected laser beam 18 is substantially orthogonal to an axis formed between the center points of cell A and cell B and is substantially parallel to strip 32. Thus, for example, if the object 14 moves from a first position, such as the one shown in FIG. 2, vertically (away from the laser diode and photodetector) in the direction of arrow Z to a second position, the point image 30 would move in the direction of arrow Y′ on the surface of photodetector 28, perpendicular to direction X and would not effect the electric output signals of the photodetector.

Additionally, in one embodiment of the invention, as shown in FIG. 2, target feature 16 has a longitudinal axis that is parallel to the plane (shown in dotted lines) formed by incident laser beam 12 and reflected laser beam 18 and is parallel to strip 32 of photodetector 28 and perpendicular to an axis formed through the center point of each cell of the bi-cell detector so that the sensor will be insensitive to minor lateral movements (in the Y direction or in the Y′ direction).

The invention advantageously provides insensitivity to movements of the object in the Z (upward), Y or Y′ (lateral in plane formed by incident and reflected light beams), and radial (about axis formed in Y direction) directions. Only movements of the object in the X (perpendicular to plane formed by incident and reflected beams) direction will be measured.

FIG. 3 is a graph showing the difference (A−B) of the amplitudes of signals A and B versus time as object 14 is moved in the direction of arrow X (shown in FIG. 2). In this example, it is assumed that object 14 is in a position at time zero such that the point image 30 is focussed off of surface 31 of bi-cell photodetector 28 (before reaching the surface of cell A). As object 14 moves in direction X, point image 30 first reaches surface 31 of cell A and then continues across strip 32 to cell B.

When the leading edge of point image (which is finite in size and appreciably larger than the width of the gap between A and B) 30 reaches the edge of cell A (after time 1), signal (A−B) begins to increase from zero, reaching its maximum value when the image is wholly on cell A (at time 2). When the leading edge of the image reaches the gap between cells A and B (at time 3), the signal (A−B) begins to decrease, passing through zero when the point image is centered on the gap between cells A and B, and lies equally on cells A and B. As the point image continues on to cell B, the signal (A−B) continues to decrease, reaching its minimum value when the image lies wholly on cell B (time 4). When the point image moves off of cell B (time 5), the signal (A−B) increases back to zero. The slope of the curve (A−B) verses image position is at a maximum when the image lies on both A and B, and increases in value as the size of the point image is reduced, while still remaining larger than the width of the gap between cells A and B.

Thus, it can be seen that by observing signal A−B, the position of point image 30 on surface 31 of bi-cell photodetector 28, and thus the linearly related relative position of object 14, can be monitored. Similarly, a displacement between a first position and a second position of object 14 can be measured.

The dynamic range (the range of positions of the object within which accurate measurements can be made) of the sensor falls within the linear range of the A−B signal curve and is shown on the graph of FIG. 3 between dotted lines labeled DR (approximately between times 2.5 and 3.5). Additionally, the system has a minimum capture range which is a minimum accurately detectable level of signal A−B. This capture range is illustrated between dotted lines labeled CR. The slope of the A−B signal curve in the linear dynamic range (in milliamps/micron) corresponds to the responsivity (or sensitivity) of the bi-cell detector. As the slope of the curve in this region increases, the resolution (the minimum detectable displacement of the object) achievable of the sensor increases but the dynamic range of the sensor decreases. Thus, it should be appreciated that with the sensor of the present invention, a balance must be struck between the dynamic range and displacement resolution.

The amplitude of signal A+B (when point image 30 is focussed on surface 31 of bi-cell detector 28) can be maintained approximately constant so that monitoring the ratio of (A−B)/(A+B) can be more simply accomplished only by monitoring signal A−B. Maintaining signal A+B constant can be accomplished by controlling the intensity of input laser beam 12 using automatic gain control (AGC) circuitry (one embodiment of which is described below with reference to FIG. 6A) electrically coupled to the laser diode 22. As described below, the AGC circuitry monitors signal A+B while controlling the power provided to the laser diode. Thus, in order to more simplify the processing circuitry necessary for monitoring the output electrical signals produced by bi-cell detector 28, an AGC circuit can be employed.

The light source 10 can be any type of light source that produces an incident light beam having sufficient intensity to be detected by the photodetector to meet the sensitivity requirements of a particular application. The light source can, for examples, include a laser diode, a light emitting diode (LED), an LED, a tungsten light source, or an optical fiber light source. If an optical fiber light source is employed, then a pair of optical fibers, or a bifurcated fiber, would be employed to direct the reflected laser beams to the photodetector. In one embodiment of the invention, the power of the light source preferably falls within the range of 0.1 milliwatt-20 milliwatt. Also, the strength of the incident laser beam preferably is approximately equal to 1 mwatt/100μ.

Various types of focussing optical elements can be employed to direct the incident and reflected light or laser beams. For example, conventional lenses, spherical or cylindrical mirrors, GRIN lenses, or molded aspheric lenses can be employed.

The photodetector can be any type of optical photodetector which produces an electric signal having a characteristic (such as amplitude or frequency, for examples) that is proportional to the position on the surface of the detector of the received light beam. The photodetector can, for example, be a position sensor in which the amplitude of the electric signal produced is directly proportional to the location on the sensor where the reflected beam is received. The photodetector preferably includes a spatially arranged photodetector capable of generating a signal or signals from which a ratio (such as (A−B)/(A+B)) can be generated. For example, the photodetector can be a bi-cell detector, a quad-cell detector, a CCD array or other.

An advantage of the sensor of the present invention is that by monitoring the ratio of (A−B)/(A+B), the sensor is somewhat insensitive to the strength of the light source, the reflectivity of the target feature and the sensitivity of the photodetector.

In one embodiment of the invention, each cell of the bi-cell detector has dimensions of 0.6 mm by 1.2 mm and the gap between the cells (the width of strip 32) is approximately equal to 10 μm. The width of image 30 can fall within the range of 50 μm-100 μm. A standard sensitivity for such photodetector is approximately equal to 0.4 amps/watt.

In a typical application, such as a servo system application (as will be described hereinafter), lens 26 may be located approximately equidistant between target feature 16 and photodetector 28, approximately 20 mm from each. Reflected beam lens 26 conventionally has a 10 mm focal length. The radius of curvature of curved surface 17 of the target feature 16 preferably falls within the range of 0.5-5.0 mm in one embodiment of the invention.

FIG. 4 is a diagram showing another embodiment of the present invention in which the optical displacement sensor is used in a disk drive servo system. As shown, the servo system conventionally includes rotatable master arm 40 (located externally of the disk drive [not shown]) and read/write head arm 42 (located within the disk drive). Both master arm 40 and head arm 42 rotate about common axis 44. Attached to the upper surface of end 54 of head arm 42 through a flexure (not shown) is a read/write head (not shown). The disk (also not shown), to which data is written and from which data is read, is located above the head arm.

As is conventional in disk drive servo systems, the master arm first is rotated to a reference position in alignment with a radial track location to be written to or read from. Then, through use of the optical displacement sensor system of the present invention, the head arm is rotated to be in alignment with the master arm so that the read/write head can read from or write to the desired radial track location of the disk.

The master arm first is aligned using an interferometric optical system. The interferometric system includes a laser source 46, a fold mirror 48, a laser interferometer 50, a retroreflector 52, attached to the underside of master arm 40, and an interferometer output signal detector 61 (shown in FIG. 5). Master arm VCM drive circuitry 63 is coupled between interferometer output signal detector 61 and voice coil motor (VCM) 65, which controls the rotational movement of master arm 40.

As should be understood, a laser interferometer employs a first laser beam, which reflects off of a moving target (i.e., retroflector 52), and a reference laser beam, which travels a fixed distance, to determine the position (or displacement) of the moving target. The laser source 46 produces a laser beam which reflects from fold mirror 48 through laser interferometer 50 to retroreflector 52. The beam is reflected by retroreflector 52 and is combined with a reference beam by interferometer 50. Interferometer 50 provides the combined beams to detector 61 which, in response, determines the displacement (or change in position) of master arm 40. Then, detector 61 sends a signal indicative of the position change to drive circuitry 63 which, in turn, provides a signal to VCM 65 to rotate master arm 40 until it is aligned with the selected radial track location.

Once the master arm 40 is properly aligned, head arm 42 then is aligned with master arm 40 using the displacement sensor of the present invention. The sensor includes source module 56 (preferably including a laser diode), reflector 58 and detector module 60 (preferably including a bi-cell photodetector), all attached to an upper surface of master arm 40. Like the general embodiment of the displacement sensor of the invention described above with reference to FIGS. 1 and 2, during operation, the laser diode within source module 56 produces an incident laser beam which is reflected by reflector 58 to a target feature (not shown) integral with, or attached to, the underside of end 54 of head arm 42. As described above, the target feature has a curved surface from which the incident laser beam is reflected to form an image of the source in close proximity of the target feature. The reflected laser beam then is reflected from reflector 58 to detector module 60. The image is refocussed on photodetector within module 60.

The system is calibrated such that when head arm 42 is properly aligned with master arm 40, a small, point-like or line-like image 30 (see FIG. 2) is focussed on the surface of dividing strip 32 between cells A and B of the bi-cell detector within detector module 60. Displacement sensor processing control circuitry 67 (shown in FIG. 5) is electrically connected to source module 56 and detector module 60 (FIG. 4) and, as will be described in more detail below, includes circuitry for determining and monitoring the values of signals A+B and A−B, which signals represent the relative positions of the head and master arms. Head arm VCM drive circuitry 69 is coupled between displacement sensor processing control circuitry 67 and head arm VCM 71. Displacement sensor processing control circuitry 67 provides a digital output signal indicative of the relative head arm and master arm positions to head arm VCM drive circuitry 69 which, in response, provides a control signal to VCM 71 to rotate head arm 42 until it is aligned with master arm 40. As will be described in greater detail below with reference to FIG. 6A, the sensor also includes AGC circuitry, coupled between the photodetector and laser diode, for maintaining constant signal A+B.

FIG. 6A shows in block diagram form, the sensor AGC circuitry of the present invention. The circuitry is shown as including bi-cell detector 62 (included within detector module 60) and laser diode 72 (included within source module 56) for description convenience. The circuitry includes conventional analog signal amplifiers 64 and 66 connected to respectively receive output signals A and B from bi-cell detector 62. Analog amplifiers 64 and 66 respectively amplify analog signals A and B. The amplified analog signals A and B are provided as outputs and also are provided to AGC circuit 73. AGC circuit 73 consists of analog summing circuit 68 and laser control power circuit 70. Summing circuit 68 produces sum signal A+B. Signal A+B is provided to laser control power circuit 70 which monitors the magnitude of sum signal A+B and, in turn, provides an output signal to control the power of laser diode 72 signal A+B is maintained substantially constant. It should be appreciated that the intensity of the incident laser beam produced by the laser diode may vary as the reflectance of the target, for example, varies, in order that signal A+B be maintained constant.

FIG. 6B shows, in block diagram form, head arm processing control circuitry 69 which controls head arm VCM 71. As shown, signals A and B are received by block 74 which includes amplifiers and adder and subtractor circuits for producing signals A−B and A+B. Signal A−B is provided to an analog-to-digital converter (ADC) 76 which converts analog signal A−B to a digital signal. The digital signal is provided to a conventional buffer 78 which provides a digital output representing signal A−B.

Signal A+B is provided to comparator 80 which compares signal A+B with a reference signal (a minimum threshold level [i.e., zero]) and provides a status output bit indicative of whether signal A+B is greater than the minimum threshold level. Circuitry 69 controls VCM 71 to rotate head arm 42 until signal A−B is equal to zero and it is determined that such zero crossing point (of signal A−B) corresponds to the point when the image point is focussed on the strip between the cells of the bi-cell detector (indicating that head arm 42 and master arm 44 are in alignment). It should be appreciated (as discussed above with reference to FIG. 3) that signal A−B also may have a value of zero when the point image is not located on the surface of either of the cells of the bi-cell detector (indicating that head arm 42 and master arm 40 are significantly out of alignment). The circuitry determines that the appropriate zero crossing is reached by monitoring signal A+B with comparator 80. With reference to FIG. 3, it should be understood that the appropriate zero crossing point of signal A−B is reached when signal A−B is equal to zero and signal A+B is greater than zero. When comparator 80 determines that signal A+B is greater than zero, a status bit is output. Thus, the digital output signal provided by buffer 78 and status bit provided by comparator 80 together indicate when head harm 42 and master arm 40 are properly aligned.

FIG. 7 shows in more detail, end 54 of head arm 42. The bottom surface of head arm 42 is shown to illustrate target feature 86. A slider flexure 84 is attached to the upper surface of head arm 42 by welding. Spot welds 82 are shown. A slider and read/write head (not shown) are connected to the tip of flexure 84. Such a construction of the flexure, slider and read/write head is conventional.

In one embodiment of the invention, the target feature 86 is formed integral with end 54 of head arm 42. The target feature can be stamped into end 54 of head arm 42 using a stamping tool that produces a curved surface preferably having a longitudinal axis parallel to the longitudinal axis of head arm. The target feature can, for example, be a cylindrical indent 88 within the head arm 42, as shown. The cylindrical indent 88 provides the curved surface of the target feature 86 that reflects the incident laser beam. The radius of curvature of the curved surface preferably falls within the range of 0.5-5.0 mm in this embodiment.

FIG. 8 is a side view of end 54 of head arm 42 showing slider 92 attached to flexure 84. As shown, slider 92 is located beneath target feature 86 at approximately the same longitudinal point along the head arm 42. Also shown is a shoulder 90 which elevates target feature 86 slightly above the remainder of head arm 42.

FIG. 9 is a part functional, part structural block diagram of a portion of the system of the invention (like FIGS. 1 and 2) illustrating the formation of the first and second images of the light source. Only a portion of the system of the invention is shown in FIG. 9. The portion shown includes target feature 16, imaging lens 26 and detector 20.

Two positions of target feature 16 are shown in FIG. 9: A first position (in which the target feature is shown in solid lines), and a second position (in which the target feature is shown in phantom). The second position shows the target feature displaced from its first position in the lateral direction X, which displacement is to be measured by the system of the invention.

As shown, incoming beam 12 reflects from curved surface 17 of target feature 16 as reflected beams 18. A small, point-like first image 100A of the light source is formed in close proximity of target feature 16. As target feature 16 moves from its first position (referred to as position A) to its second position (referred to as position B), the first point-like image also moves with target feature 16 from its first position (denoted as 100A) to its second position (denoted as 100B).

At each position, a portion of the reflected beams 18 is received by imaging lens 26 and focused onto a surface of photodetector 20. For position A, a second image 100A′ is formed on surface of photodetector 20. For position B, a second image 100B′ is formed on surface of photodetector 20. As can be seen, as target feature 16 moves from position A to position B in lateral direction X, second point-like image moves from position 100A′ to position 100B′ (which corresponds to movement in the X′ direction of image 30 on surface 31 of photodetector 20 in FIG. 2). It should be appreciated that second image 100A′ (and 100B′) corresponds to second image 30 in FIG. 2.

In the servo system embodiment of the invention (shown in FIG. 4), only rotational movements of the head arm about axis 44 (which corresponds to movements of object 14 in the X direction in FIG. 2) will be measured. None of radial movements of the target feature with respect to the head arm (corresponding to rotation of object 14 about axis Y in FIG. 2), movements of the head arm vertically away from or toward the master arm (corresponding movements in direction Z in FIG. 2), or movements of the target feature in either direction formed by a longitudinal axis of the head arm (corresponding to movements in direction Y in FIG. 2), will be detected by the system.

Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. For example, while the sensor of the invention was described as one which monitors the position in one dimension (or direction) of an object, by employing a bi-cell detector, it should be appreciated that the invention is not so limited. By employing a quad-cell detector, the sensor could respectively monitor the position of an object in two dimensions. Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto. 

What is claimed is:
 1. A device for measuring the displacement of a movable object in a desired direction comprising: a stationary light source that produces an incident light beam; a target feature, attached to the object, that reflects the incident light beam and forms a first image of the light source in close proximity of the target feature; an imaging lens, that receives the first image and reforms the first image as a second image of the light source on a photodetector; and the photodetector, spaced from the object, that detects a linear displacement in a first direction of the second image; wherein movement of the object in the desired direction corresponds to movement of the second image in the first direction such that the device is relatively insensitive to movements of the object in directions other than the desired direction.
 2. The device as claimed in claim 1 wherein the photodetector includes a bi-cell photodetector.
 3. The device as claimed in claim 1 wherein the photodetector includes a position sensor.
 4. The device as claimed in claim 1 wherein the target feature is integral with the object.
 5. The device as claimed in claim 1 wherein the target feature has a radius of curvature within the range of 0.2-5.0 mm.
 6. The device as claimed in claim 1 wherein the incident light beam and the reflected light beam form a plane perpendicular to the desired direction.
 7. The device as claimed in claim 2 wherein an axis formed through a center point of each cell of the bi-cell detector is substantially parallel to the desired direction and is substantially orthogonal to a plane formed by the incident light beam and the reflected light beam.
 8. An optical position sensing device for sensing a rotational position of a read/write head arm relative to that of a master arm in a disk drive, the device comprising: a light source, attached to the master arm, that produces an incident light beam; a target feature, attached to the head arm, that reflects the incident light beam and forms a first image of the light source in close proximity of the target feature; an imaging lens located in a light path between the target feature and a photodetector, that receives the reflected light beam, and refocusses the first image of the light source as a second image on the photodetector; and the photodetector, attached to the master arm, that detects a linear displacement in a first direction of the second image; wherein relative rotational movements of the head arm and/or master arm correspond to movement of the second image in the first direction such that the device is relatively insensitive to movements of the target feature other than those caused by relative rotational movements of the head arm and/or master arm.
 9. The device as claimed in claim 8 wherein the photodetector includes a bi-cell photodetector.
 10. The device as claimed in claim 8 wherein the photodetector includes a position-sensing sensor.
 11. The device as claimed in claim 8 wherein the target feature is integral with the head arm.
 12. The device as claimed in claim 8 wherein the target feature has a radius of curvature within the range of 0.2-5.0 mm.
 13. The device as claimed in claim 8 wherein the incident light beam and the reflected light beam form a plane perpendicular to the desired direction.
 14. The device as claimed in claim 9 wherein an axis formed through a center point of each cell of the bi-cell detector is substantially parallel to a direction of motion of the head arm and is substantially orthogonal to a plane formed by the incident light beam and the reflected light beam.
 15. The device as claimed in claim 8 wherein the target feature and a read/write head are located adjacent a tip of the head arm.
 16. A device as claimed in claim 15 wherein the target feature and read/write head are located on opposite sides of the head arm.
 17. A device as claimed in claim 8 wherein the head arm and the master arm share a common axis of rotation.
 18. A method of measuring the displacement in a desired direction of a movable object comprising the steps of: producing an incident light beam with a stationary light source; reflecting the incident light beam from a curved surface of a target feature, attached to the object, to form a first image of the light source in close proximity of the target feature; refocussing the first image of the light source as a second image onto a stationary photodetector; and detecting, with the photodetector, a linear displacement of the second image in a first direction which corresponds to displacement of the object in the desired direction. 